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Synergistic effect of high hydrostatic pressure pretreatment and osmotic stress on mass transfer during osmotic dehydration
Journal of Food Engineering 45 (2000) 25±31
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Synergistic eect of high hydrostatic pressure pretreatment and osmotic stress on mass transfer during osmotic dehydration N.K. Rastogi 1, A. Angersbach, D. Knorr * Department of Food Biotechnology and Food Process Engineering, Berlin University of Technology, K onigin-Luise-Strasse 22, D-14195 Berlin, Germany Received 28 October 1999; accepted 24 January 2000
Abstract During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with the surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cell permeabilisation. The cell permeabilisation index (Zp , as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. Osmotic dehydration of HHP-treated foods is faster than that of untreated foods. The state of the cell membrane during osmotic dehydration of high-pressure-pretreated samples can change from being partially to totally permeable, which leads to signi®cant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: High hydrostatic pressure; Osmotic dehydration; Mass transfer
1. Introduction Osmotic dehydration is widely used for the partial removal of water from plant tissues by immersion in a hypertonic (osmotic) solution. The driving force for the diusion of water from the tissue into the solution is provided by the higher osmotic pressure of the hypertonic solution. The diusion of water is accompanied by the simultaneous counter diusion of solute from the osmotic solution into the tissue. Since the membrane responsible for osmotic transport is not perfectly selective, other solutes present in the cells can also be leached into the osmotic solution (Dixon & Jen, 1977; Lerici, Pinnavaia, Dalla Rosa & Bartolucci, 1985; Giangiacomo, Torreggiani & Abbo, 1987). The rate of diusion of water from any material made up of such tissues depends upon factors such as: temperature and concentration of the osmotic solution, the size and geometry of the material, the solution to material mass ratio and the level of agitation of the solution. A number of recent * Corresponding author. Tel.: +49-30-314-71250; fax: +49-30-8327663. E-mail address: [email protected] (D. Knorr). 1 On DAAD fellowship from: Department of Food Engineering, Central Food Technological Research Institute, Mysore 570 013, India.
publications have described the in¯uence of these variables on mass transfer rates during osmotic dehydration (Roult-Wack, Lenart & Guilbert, 1992; Torreggiani, 1993; Roult-Wack, 1994; Rastogi & Raghavarao, 1994, 1995, 1997a,b; Rastogi, Raghavarao & Niranjan, 1997). Given the low rate of mass transfer observed, a number of techniques have also been tried to improve mass transfer rate. These techniques include: the application of a partial vacuum (Rastogi & Raghavarao, 1996; Fito, 1994) or ultrasound during treatment (Simal, Benedito, Sanchez & Rossello, 1999) and ultra high hydrostatic pressure (Rastogi & Niranjan, 1998) or high intensity electrical ®eld pulses (Rastogi, Eshtiaghi & Knorr, 1999) to the material prior to osmotic treatment. Mass transfer during osmotic treatment occurs through semi-permeable cell membranes present in biological materials, which oers the dominant resistance to the process. The state of the cell membrane can change from being partially to being totally permeable and this can lead to signi®cant changes in tissue architecture. During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. The most likely cause of cell damage can be attributed to the reduction in size caused by water loss during osmotic treatment, which results in
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 3 7 - 6
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N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
the loss of contact between cell membrane and the cell wall. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cells disintegration. The cell disintegration index (Zp , as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. It is well known that application of high pressures (100±800 MPa) causes permeabilisation of the cell structure (Farr, 1990; Dornenburg & Knorr, 1993; Rastogi, Subramanian & Raghavarao, 1994; Eshtiaghi, Stute & Knorr, 1994). This phenomenon is exploited here to enhance mass transfer rates during osmotic dehydration, by pretreating the samples at such high pressures. Osmotic dehydration of HHP treated pineapple has been reported to be faster than untreated ones (Rastogi & Niranjan, 1998). The application of high pressure damages the cell wall structure, leaving the cells more permeable, which leads to signi®cant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples. The main purposes of the present work are: (1) to investigate the combined eect of high hydrostatic pressure and osmotic stress on the extent of membranes permeabilisation; (2) to explain mass transfer mechanism of mass transfer during osmotic removal of water from high-pressure-treated foods considering potato as a model homogeneous plant material. 2. Materials and methods 2.1. Materials 2.1.1. Potato Fresh potatoes were purchased from a local super market. The average moisture content of the potato was 86.5%. 2.2. Methods 2.2.1. Examination of cell condition by an electro-physical method The cell condition was examined in terms of an index, Zp . The method is based on electrical impedance analysis of biological systems in the frequency and ranges typically between 1 kHz and 100 MHz of b-dispersion (Shaw & Galvin, 1949; Schwan, 1977; Angersbach, Heinz & Knorr, 1997). A homogeneous distribution of intact and permeabilised cells, and non cellular compartments in the measured system is a prerequisite for correct measurement. Zp is an integral parameter which indicates the relative reduction in the portion of intact cells in the cell system. During osmotic treatment, the reduction in cells can occur due to disintegration or
shrinkage of cell membranes, Zp is determined as follows (Angersbach et al., 1997; Angersbach, Heinz & Knorr, 1999) Zp
rih =rth rtl ÿ ril ; rih ÿ ril
where r is the electrical conductivity, the superscripts i and t refer to conductivities before and after osmotic treatment, respectively, and subscripts l and h refer to the low and high frequency within the b-dispersion band. For potato cells, the characteristic low and high frequencies were 3 kHz and 6.25 MHz, respectively. The value of Zp ranges between 0 (for intact cells system) and 1 (for complete membrane rupture). The conductivity was measured using parallel plate disk electrodes (9.7 mm diameter). The distance between the electrodes was 8 mm. A correction for the capacitance and inductance (of about 1.2 lH) arising from the measuring cell was incorporated using Schwan`s method (Schwan, 1963, 1968). The cell disintegration index (Zp ) values were measured immediately after the high pressure treatment. The variation of Zp values with time was also measured for the samples treated at 200 and 400 MPa. 2.2.2. High pressure pretreatment The potato slices (thickness 1 cm) were subjected to high hydrostatic pressures in a pressure vessel (National Forge Europe, Belgium). The unit had a working volume of 700 ml and a maximum recommended working pressure of 600 MPa. The pressure could built up within 2 min and the decompression time was about 10 s. Potato samples were vacuum-sealed in double polyethylene pouches. The samples were subjected to 200 and 400 MPa for 10 min at 25°C. The maximum temperature experienced by the sample during pressurisation was 30°C and it cooled to about 15°C during decompression. A mixture of distilled water and press oil (97:3 v/v) was used as a medium for transmitting pressure. 2.2.3. Osmotic treatment High-pressure-treated and untreated potato sheets (thickness 1 cm) were subjected to osmotic treatment after removing adhering water with a tissue. The samples were weighed and suspended in the vessel containing the sugar solution (50°Brix). The temperature of the sugar solution was maintained at 25°C using a constant temperature-stirred water bath. The sugar solution was continuously agitated. The ratio of the volume of the pieces to that of the medium was maintained at 1:25 in order to ensure that the concentration of the osmotic solution did not change signi®cantly during the experiment. Samples were withdrawn at regular intervals of 1 h, rinsed quickly, and wiped gently with a tissue paper. The potato sheet after osmotic treatment was cut longitudinally into nine slices, 1 mm length, using sharp
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
blades mounted on a screw, and used to determine the moisture content and Zp values with respect to the distance from the centre of the sample. The samples were then weighed and dried in a vacuum oven at 70°C for 18 h. The moisture and solid content was expressed in terms of kg of water/kg of initial dry solids and kg of solid/kg of initial dry solids, respectively. All the experiments were done in triplicate and average values have been reported. Relative moisture and solid content are as follows: Relative moisture content
Moisture content at any time M Initial moisture content M0
Solid content at any time S Relative solid content Initial soild content S0 2.2.4. Determination of water and solute diusion coef®cients The solution of FickÕs second law for diusion from an in®nite slab (thickness 2 l) being dehydrated from both the sides, with suitable assumptions and boundary conditions, can be written for diused moisture (Mr ) and solid ratio (Sr ) as (Crank, 1975): Mr
mt ÿ m1 m 0 ÿ m 1
" # 2 1 8X 1 1 2 exp ÿ n p Fow 2 p n0 2n 12 2
1
and Sr
st ÿ s1 s0 ÿ s1
" # 2 1 8X 1 1 2 exp ÿ n p Fos ; 2 p n0 2n 12 2
2
where Fow and Fos are the Fourier numbers for moisture and solid diusion and can be de®ned as Dew t/l2 and Des t/l2 , respectively, where Dew and Des are the eective diusion coecients for water and solute; t is the immersion time and l is the half thickness of the in®nite slab. The values of Fow and Fos were obtained from Eqs. (1) and (2). These values are then plotted against the corresponding values of t and the Dew and Des values were estimated from the slopes of these plots (Rastogi & Raghavarao, 1997a,b). 3. Results and discussion 3.1. Osmotic dehydration under osmotic stress The movement of the dehydration front during osmotic dehydration towards the centre of the material resulted in cell membrane disintegration in the dehy-
27
Fig. 1. Mechanism of mass transfer during osmotic dehydration.
drated region and the water is transported across three dierent regions, each with its own characteristic properties: diusion of water from the core of the material to the dehydration front, diusion of water across the front and diusion of water through the osmotically treated material into the surrounding medium (Rastogi, Angersbach & Knorr, 1999). The mechanisms of osmotic water removal from biological materials can be explained with the help of Fig. 1. First water is diused from the outer layer of the sample to the osmotic medium, thereby increasing the osmotic pressure at the surface. As the osmotic pressure reached a critical value, the cell membranes are ruptured and shrunk. This resulted in a steep reduction in the proportion of intact cells (i.e., an increase in Zp value). At this point, the rate of mass transfer increased sharply and a relatively large amount of water is diused out with a diusion coecient D2 , (D2 D3 ). At any point of time, D3 is the diusion coecient of water released through the ruptured and shrunk cells into osmotic solution. The diusion coecient of water from the core of the material (D1 ) is much lower in comparison to D2 and D3 . The pro®les for relative moisture content (M/M0 ) and cell disintegration index (Zp ) values have also been indicated (Fig. 1). The distribution of cell disintegration index (Zp ) values as well as relative moisture (M/M0 ) and solid (S/S0 ) content (Figs. 4±6) show that the rate of change of moisture and solid content was very high at the interface and it decreased towards the centre. Similarly, as the transition layer (dehydration front) moved towards the centre, it resulted in a sudden increase in Zp values. 3.2. Combined eect of osmotic stress and high-pressureinduced permeabilisation Fig. 2(a) and (b) show the variation in moisture and solid content of control and high pressure-treated samples with immersion time during the course of osmotic dehydration. The moisture and solid diusion both increased due to high-pressure pretreatment. It also increased with an increase in pretreatment pressure. In order to compare the mass transfer characteristics, the
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N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
Fig. 2. Variation of: (a) moisture and (b) solid content of control and high-pressure-treated potato sample during the course of osmotic dehydration.
eective diusion coecients of water diusing out of the sample and solute infusing into the sample were calculated. The relevant values are reported in Table 1. It is evident that the diusion coecient for water was increased by high pressure pretreatment, giving shorter dehydration times. There was also a simultaneous increase in solute diusion coecients. This is attributed to the increase in cell permeability due to high pressure pretreatment. The cell permeation index (Zp ) is a function of high hydrostatic pressure applied to the potato sample (Fig. 3(a)). The Zp values were recorded immediately Table 1 Eective diusion coecient for water (Dew ) and solute (Des ) at different pressure Treatment pressure (MPa)
Dew 109 (m2 /s)
Des 109 (m2 /s)
Control 200 400
0.37 0.45 0.84
0.31 0.41 0.67
Fig. 3. (a) Variation of cell disintegration index (Zp ) with applied pressure. The Zp values were measured immediately after high pressure treatment; (b) Variation of cell disintegration index (Zp ) with time at atmospheric pressure after high pressure treatment.
after the high pressure treatment. The Zp values of the pressure-treated potato increased with time at atmospheric pressure and the equilibrium Zp values also increased with pressure (Fig. 3(b)). The Zp values of the centre layer after high pressure pretreatment are 0.039 and 0.091 at 200 and 400 MPa, respectively and can reach 0.35 and 0.53 within 6 h (Fig. 4). The further increase in Zp values of the centre layer (upto 0.82) after 5 h in case of samples treated at 400 MPa is due to the combined eect of osmotic stress and high-pressure-induced permeabilisation (Fig. 4(c)). The increase in Zp values (or tissue softening or loss of texture) following high pressure treatment was due to destruction of cell membranes and partial liberation of cell substances. Upon high pressure treatment, polymethylesterase (PME) enzyme is liberated and not completely inactivated (which is bound to the cell wall) and brought in close contact with its substrate, the methylated pectin. This caused de-esteri®cation not only during high pressure treatment but also after the release of high pressure (standing time). The pressure treatment also caused partial inactivation of PME. This reaction continued with time even after high pressure treatment
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
29
Fig. 4. Distribution of cell disintegration index with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
Fig. 5. Distribution of relative moisture content with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
and results in time-depended softening of potato tissue. The alteration in pectin resulted in loss of water and soluble solids (or extractable pectin) after high pressure treatment (Stute, Eshtiaghi, Boguslawski & Knorr, 1996). It is reported in the literature that the softening of some fruits and vegetables at atmospheric pressure over a length of time takes place following high pressure treatment (Basak & Ramaswamy, 1998). The variation in cell disintegration index (Zp ) of pressure-treated and untreated potato samples with distance from the centre of the material for dierent dehydration times, is shown in Fig. 4. It may be inferred
from the ®gures that as the dehydration front moved within the sample, it resulted in sudden increases in Zp values. The pro®les of cell disintegration index (Zp ) were dierent for control and the samples treated at 200 and 400 MPa, due to dierence in the degree of cell disintegration with the applied pressure (Fig. 3(a) and (b)). The Zp values of the untreated sample did not change with time and remained zero. The Zp values of the centre layer after 5 h, for the control and sample treated at 200 MPa were 0.18 and 0.28, respectively. In case of samples treated at 400 MPa, the Zp values were 0.82, which showed that the variation in Zp values was due to
30
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
combined eect of osmotic stress owing to osmotic pressure as the dehydration proceeds and high-pressureinduced permeabilisation. The Zp values of all the layers, in the case of pressure treated at 400 MPa, were higher than the control and the sample treated at 200 MPa at the corresponding distances and immersion times. There was a progressive increase in the Zp values as the osmotic dehydration proceeded. The Zp values of the centre layer of potato treated at 400 MPa after 5 h reached 0.24, 0.50, 0.66, 0.75 and 0.82 after 1, 2, 3, 4 and 5 h, respectively (Fig. 4).
The variations in relative moisture (M/M0 ) and solid (S/S0 ) content with respect to the distance from the centre of the material for the pressure treated and control potato samples for dierent treatment times are shown in Figs. 5 and 6. The relative moisture and solid content for control and the sample treated at 200 MPa were not signi®cantly dierent, whereas the moisture content was reduced and solid content increased in the case of samples treated at 400 MPa.
4. Conclusion The cell disintegration index (Zp ) values were close to 1 (total permeabilisation) at the surface in contact with surrounding osmotic solution and as the transition layer (dehydration front) moved towards the centre, it resulted in a sudden increase in Zp values. In case of HHPtreated samples, the Zp values were also high at the centre (which increased with time) which is due to combined eect of high hydrostatic pressure and osmotic stress. The application of high pressure resulted in cell permeabilisation facilitating diusion. The osmotic dehydration of high pressure-treated potato samples was faster than the untreated one due to the combined eect of cell permeabilisation due to osmotic stress (as the dehydration proceeds) and high-pressure-induced permeabilisation.
Acknowledgements The author NKR gratefully acknowledges the support of German Academic Exchange Service (DAAD) for providing a research fellowship at Berlin University of Technology, Germany.
References
Fig. 6. Distribution of relative solid content with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
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N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31 Eshtiaghi, M. N., Stute, R., & Knorr, D. (1994). High pressure and freezing pretreatment eects on drying rehydration, texture and colour of green beans, carrots and potatoes. Journal of Food Science, 59, 1168±1170. Farr, D. (1990). High pressure technology in food industry. Trends in Food Science and Technology, 1, 14±16. Fito, P. (1994). Modelling of vacuum osmotic dehydration of food. Journal of Food Engineering, 22, 313±328. Giangiacomo, R., Torreggiani, D., & Abbo, E. (1987). Osmotic dehydration of fruit. Part I: Sugar exchange between fruit and extracting syrup. Journal of Food Processing and Preservation, 11, 183±195. Lerici, C. L., Pinnavaia, G., Dalla Rosa, M., & Bartolucci, L. (1985). Osmotic dehydration of fruit: In¯uence of osmotic agents on drying behaviour an product quality. Journal of Food Science, 50, 1217±1219. Rastogi, N. K., & Raghavarao, K. S. M. S. (1994). Eect of temperature and concentration of osmotic dehydration of coconut. Lebensmittel-Wissenchaft-Technologie (LWT), 27, 264±567. Rastogi, N. K., Subramanian, R., & Raghavarao, K. S. M. S. (1994). Application of high pressure technology in food industry. Indian Food Industry, 13, 30±34. Rastogi, N. K., & Raghavarao, K. S. M. S. (1995). Kinetics of osmotic dehydration of coconut. Journal of Food Process Engineering, 18, 187±197. Rastogi, N. K., & Raghavarao, K. S. M. S. (1996). Kinetics of osmotic dehydration under vacuum. Lebensmittel-Wissenchaft-Technologie (LWT), 29, 669±672. Rastogi, N. K., Raghavarao, K. S. M. S., & Niranjan, K. (1997). Mass transfer during osmotic dehydration of banana: Fickian diusion in cylindrical con®guration. Journal of Food Engineering, 31, 423±432. Rastogi, N. K., & Raghavarao, K. S. M. S. (1997a). Water and solute diusion coecients of carrot as a function of temperature and concentration. Journal of Food Engineering, 34, 429±440. Rastogi, N. K., & Raghavarao, K. S. M. S. (1997b). Mass transfer during osmotic dehydration of carrot: Comparison of dierent methods for the estimation of eective diusion coecients. In
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Proceedings of Seventh International Congress on Engineering and Food (ICEF). Brighton, UK (Vol. 2, G 73±76). Rastogi, N. K., & Niranjan, K. (1998). Enhanced mass transfer during osmotic dehydration of high-pressure-treated pineapple. Journal of Food Science, 63 (3), 508±511. Rastogi, N. K., Eshtiaghi, M. N., & Knorr, D. (1999). Accelerated mass transfer during osmotic dehydration of high intensity electrical ®eld pulse pretreated carrots. Journal of Food Science, 64(6), 1020±1023. Rastogi, N. K., Angersbach, A., & Knorr, D. (1999). Evaluation of mass transfer mechanisms during osmotic treatment of plant materials. Journal of Food Science, forwarded. Roult-Wack, A. L., Lenart, A., & Guilbert, S. (1992). Recent advances during dewatering through immersion in concentrated solution. In A. S. Majumdar, Drying of solids (pp. 21±51). New York: International Science Publisher. Roult-Wack, A. L. (1994). Advances in osmotic dehydration. Trends in Food Science and Technology, 5, 255±260. Schwan, H. P. (1963). Determination of biological impedance. In W. L. Nastuk, Physical techniques in biological research (Vol. 6, p. 323). New York: Academic Press. Schwan, H. P. (1968). Electrode polarization impedance and measurements in biological materials. Annals of New York Academy of Science, 148, 191±209. Schwan, H. P. (1977). Field interaction with biological matter. Annals of New York Academy of Science, 303, 198±213. Shaw, T. M., & Galvin, J. A. (1949). High-frequency-heating characteristics of vegetable tissues determined from electrical-conductivity measurements. In Proceedings of the I.R.E. ±Waves and Electrons Section (Vol. 37, pp. 83±86). Simal, S., Benedito, J., Sanchez, E. S., & Rossello, C. (1999). Use of ultrasound to increase mass transport rates during osmotic dehydration. Journal of Food Engineering, 36, 323±336. Stute, R., Eshtiaghi, M. N., Boguslawski, S. & Knorr, D. (1996). High pressure treatment of vegetables. In Ph. Rudolf Von Rohr, Ch. Trepp, High pressure chemical engineering. New York: Elsevier. Torregginni, D. (1993). Osmotic dehydration in fruits and vegetable processing. Food Research International, 26, 59±68.
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Synergistic eect of high hydrostatic pressure pretreatment and osmotic stress on mass transfer during osmotic dehydration N.K. Rastogi 1, A. Angersbach, D. Knorr * Department of Food Biotechnology and Food Process Engineering, Berlin University of Technology, K onigin-Luise-Strasse 22, D-14195 Berlin, Germany Received 28 October 1999; accepted 24 January 2000
Abstract During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with the surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cell permeabilisation. The cell permeabilisation index (Zp , as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. Osmotic dehydration of HHP-treated foods is faster than that of untreated foods. The state of the cell membrane during osmotic dehydration of high-pressure-pretreated samples can change from being partially to totally permeable, which leads to signi®cant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: High hydrostatic pressure; Osmotic dehydration; Mass transfer
1. Introduction Osmotic dehydration is widely used for the partial removal of water from plant tissues by immersion in a hypertonic (osmotic) solution. The driving force for the diusion of water from the tissue into the solution is provided by the higher osmotic pressure of the hypertonic solution. The diusion of water is accompanied by the simultaneous counter diusion of solute from the osmotic solution into the tissue. Since the membrane responsible for osmotic transport is not perfectly selective, other solutes present in the cells can also be leached into the osmotic solution (Dixon & Jen, 1977; Lerici, Pinnavaia, Dalla Rosa & Bartolucci, 1985; Giangiacomo, Torreggiani & Abbo, 1987). The rate of diusion of water from any material made up of such tissues depends upon factors such as: temperature and concentration of the osmotic solution, the size and geometry of the material, the solution to material mass ratio and the level of agitation of the solution. A number of recent * Corresponding author. Tel.: +49-30-314-71250; fax: +49-30-8327663. E-mail address: [email protected] (D. Knorr). 1 On DAAD fellowship from: Department of Food Engineering, Central Food Technological Research Institute, Mysore 570 013, India.
publications have described the in¯uence of these variables on mass transfer rates during osmotic dehydration (Roult-Wack, Lenart & Guilbert, 1992; Torreggiani, 1993; Roult-Wack, 1994; Rastogi & Raghavarao, 1994, 1995, 1997a,b; Rastogi, Raghavarao & Niranjan, 1997). Given the low rate of mass transfer observed, a number of techniques have also been tried to improve mass transfer rate. These techniques include: the application of a partial vacuum (Rastogi & Raghavarao, 1996; Fito, 1994) or ultrasound during treatment (Simal, Benedito, Sanchez & Rossello, 1999) and ultra high hydrostatic pressure (Rastogi & Niranjan, 1998) or high intensity electrical ®eld pulses (Rastogi, Eshtiaghi & Knorr, 1999) to the material prior to osmotic treatment. Mass transfer during osmotic treatment occurs through semi-permeable cell membranes present in biological materials, which oers the dominant resistance to the process. The state of the cell membrane can change from being partially to being totally permeable and this can lead to signi®cant changes in tissue architecture. During osmotic removal of water from foods, the osmotic dehydration front moves from the surface of the food in contact with surrounding osmotic solution to the centre, which results in disintegration of cells due to osmotic stress. The most likely cause of cell damage can be attributed to the reduction in size caused by water loss during osmotic treatment, which results in
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 0 0 ) 0 0 0 3 7 - 6
26
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
the loss of contact between cell membrane and the cell wall. When the food is pretreated with high hydrostatic pressure (HHP), it also results in cells disintegration. The cell disintegration index (Zp , as measured by an electrophysical measurement based on electrical impedance analysis) after high pressure treatment increases with time. It is well known that application of high pressures (100±800 MPa) causes permeabilisation of the cell structure (Farr, 1990; Dornenburg & Knorr, 1993; Rastogi, Subramanian & Raghavarao, 1994; Eshtiaghi, Stute & Knorr, 1994). This phenomenon is exploited here to enhance mass transfer rates during osmotic dehydration, by pretreating the samples at such high pressures. Osmotic dehydration of HHP treated pineapple has been reported to be faster than untreated ones (Rastogi & Niranjan, 1998). The application of high pressure damages the cell wall structure, leaving the cells more permeable, which leads to signi®cant changes in the tissue architecture resulting in increased mass transfer rates during osmotic dehydration as compared to untreated samples. The main purposes of the present work are: (1) to investigate the combined eect of high hydrostatic pressure and osmotic stress on the extent of membranes permeabilisation; (2) to explain mass transfer mechanism of mass transfer during osmotic removal of water from high-pressure-treated foods considering potato as a model homogeneous plant material. 2. Materials and methods 2.1. Materials 2.1.1. Potato Fresh potatoes were purchased from a local super market. The average moisture content of the potato was 86.5%. 2.2. Methods 2.2.1. Examination of cell condition by an electro-physical method The cell condition was examined in terms of an index, Zp . The method is based on electrical impedance analysis of biological systems in the frequency and ranges typically between 1 kHz and 100 MHz of b-dispersion (Shaw & Galvin, 1949; Schwan, 1977; Angersbach, Heinz & Knorr, 1997). A homogeneous distribution of intact and permeabilised cells, and non cellular compartments in the measured system is a prerequisite for correct measurement. Zp is an integral parameter which indicates the relative reduction in the portion of intact cells in the cell system. During osmotic treatment, the reduction in cells can occur due to disintegration or
shrinkage of cell membranes, Zp is determined as follows (Angersbach et al., 1997; Angersbach, Heinz & Knorr, 1999) Zp
rih =rth rtl ÿ ril ; rih ÿ ril
where r is the electrical conductivity, the superscripts i and t refer to conductivities before and after osmotic treatment, respectively, and subscripts l and h refer to the low and high frequency within the b-dispersion band. For potato cells, the characteristic low and high frequencies were 3 kHz and 6.25 MHz, respectively. The value of Zp ranges between 0 (for intact cells system) and 1 (for complete membrane rupture). The conductivity was measured using parallel plate disk electrodes (9.7 mm diameter). The distance between the electrodes was 8 mm. A correction for the capacitance and inductance (of about 1.2 lH) arising from the measuring cell was incorporated using Schwan`s method (Schwan, 1963, 1968). The cell disintegration index (Zp ) values were measured immediately after the high pressure treatment. The variation of Zp values with time was also measured for the samples treated at 200 and 400 MPa. 2.2.2. High pressure pretreatment The potato slices (thickness 1 cm) were subjected to high hydrostatic pressures in a pressure vessel (National Forge Europe, Belgium). The unit had a working volume of 700 ml and a maximum recommended working pressure of 600 MPa. The pressure could built up within 2 min and the decompression time was about 10 s. Potato samples were vacuum-sealed in double polyethylene pouches. The samples were subjected to 200 and 400 MPa for 10 min at 25°C. The maximum temperature experienced by the sample during pressurisation was 30°C and it cooled to about 15°C during decompression. A mixture of distilled water and press oil (97:3 v/v) was used as a medium for transmitting pressure. 2.2.3. Osmotic treatment High-pressure-treated and untreated potato sheets (thickness 1 cm) were subjected to osmotic treatment after removing adhering water with a tissue. The samples were weighed and suspended in the vessel containing the sugar solution (50°Brix). The temperature of the sugar solution was maintained at 25°C using a constant temperature-stirred water bath. The sugar solution was continuously agitated. The ratio of the volume of the pieces to that of the medium was maintained at 1:25 in order to ensure that the concentration of the osmotic solution did not change signi®cantly during the experiment. Samples were withdrawn at regular intervals of 1 h, rinsed quickly, and wiped gently with a tissue paper. The potato sheet after osmotic treatment was cut longitudinally into nine slices, 1 mm length, using sharp
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
blades mounted on a screw, and used to determine the moisture content and Zp values with respect to the distance from the centre of the sample. The samples were then weighed and dried in a vacuum oven at 70°C for 18 h. The moisture and solid content was expressed in terms of kg of water/kg of initial dry solids and kg of solid/kg of initial dry solids, respectively. All the experiments were done in triplicate and average values have been reported. Relative moisture and solid content are as follows: Relative moisture content
Moisture content at any time M Initial moisture content M0
Solid content at any time S Relative solid content Initial soild content S0 2.2.4. Determination of water and solute diusion coef®cients The solution of FickÕs second law for diusion from an in®nite slab (thickness 2 l) being dehydrated from both the sides, with suitable assumptions and boundary conditions, can be written for diused moisture (Mr ) and solid ratio (Sr ) as (Crank, 1975): Mr
mt ÿ m1 m 0 ÿ m 1
" # 2 1 8X 1 1 2 exp ÿ n p Fow 2 p n0 2n 12 2
1
and Sr
st ÿ s1 s0 ÿ s1
" # 2 1 8X 1 1 2 exp ÿ n p Fos ; 2 p n0 2n 12 2
2
where Fow and Fos are the Fourier numbers for moisture and solid diusion and can be de®ned as Dew t/l2 and Des t/l2 , respectively, where Dew and Des are the eective diusion coecients for water and solute; t is the immersion time and l is the half thickness of the in®nite slab. The values of Fow and Fos were obtained from Eqs. (1) and (2). These values are then plotted against the corresponding values of t and the Dew and Des values were estimated from the slopes of these plots (Rastogi & Raghavarao, 1997a,b). 3. Results and discussion 3.1. Osmotic dehydration under osmotic stress The movement of the dehydration front during osmotic dehydration towards the centre of the material resulted in cell membrane disintegration in the dehy-
27
Fig. 1. Mechanism of mass transfer during osmotic dehydration.
drated region and the water is transported across three dierent regions, each with its own characteristic properties: diusion of water from the core of the material to the dehydration front, diusion of water across the front and diusion of water through the osmotically treated material into the surrounding medium (Rastogi, Angersbach & Knorr, 1999). The mechanisms of osmotic water removal from biological materials can be explained with the help of Fig. 1. First water is diused from the outer layer of the sample to the osmotic medium, thereby increasing the osmotic pressure at the surface. As the osmotic pressure reached a critical value, the cell membranes are ruptured and shrunk. This resulted in a steep reduction in the proportion of intact cells (i.e., an increase in Zp value). At this point, the rate of mass transfer increased sharply and a relatively large amount of water is diused out with a diusion coecient D2 , (D2 D3 ). At any point of time, D3 is the diusion coecient of water released through the ruptured and shrunk cells into osmotic solution. The diusion coecient of water from the core of the material (D1 ) is much lower in comparison to D2 and D3 . The pro®les for relative moisture content (M/M0 ) and cell disintegration index (Zp ) values have also been indicated (Fig. 1). The distribution of cell disintegration index (Zp ) values as well as relative moisture (M/M0 ) and solid (S/S0 ) content (Figs. 4±6) show that the rate of change of moisture and solid content was very high at the interface and it decreased towards the centre. Similarly, as the transition layer (dehydration front) moved towards the centre, it resulted in a sudden increase in Zp values. 3.2. Combined eect of osmotic stress and high-pressureinduced permeabilisation Fig. 2(a) and (b) show the variation in moisture and solid content of control and high pressure-treated samples with immersion time during the course of osmotic dehydration. The moisture and solid diusion both increased due to high-pressure pretreatment. It also increased with an increase in pretreatment pressure. In order to compare the mass transfer characteristics, the
28
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
Fig. 2. Variation of: (a) moisture and (b) solid content of control and high-pressure-treated potato sample during the course of osmotic dehydration.
eective diusion coecients of water diusing out of the sample and solute infusing into the sample were calculated. The relevant values are reported in Table 1. It is evident that the diusion coecient for water was increased by high pressure pretreatment, giving shorter dehydration times. There was also a simultaneous increase in solute diusion coecients. This is attributed to the increase in cell permeability due to high pressure pretreatment. The cell permeation index (Zp ) is a function of high hydrostatic pressure applied to the potato sample (Fig. 3(a)). The Zp values were recorded immediately Table 1 Eective diusion coecient for water (Dew ) and solute (Des ) at different pressure Treatment pressure (MPa)
Dew 109 (m2 /s)
Des 109 (m2 /s)
Control 200 400
0.37 0.45 0.84
0.31 0.41 0.67
Fig. 3. (a) Variation of cell disintegration index (Zp ) with applied pressure. The Zp values were measured immediately after high pressure treatment; (b) Variation of cell disintegration index (Zp ) with time at atmospheric pressure after high pressure treatment.
after the high pressure treatment. The Zp values of the pressure-treated potato increased with time at atmospheric pressure and the equilibrium Zp values also increased with pressure (Fig. 3(b)). The Zp values of the centre layer after high pressure pretreatment are 0.039 and 0.091 at 200 and 400 MPa, respectively and can reach 0.35 and 0.53 within 6 h (Fig. 4). The further increase in Zp values of the centre layer (upto 0.82) after 5 h in case of samples treated at 400 MPa is due to the combined eect of osmotic stress and high-pressure-induced permeabilisation (Fig. 4(c)). The increase in Zp values (or tissue softening or loss of texture) following high pressure treatment was due to destruction of cell membranes and partial liberation of cell substances. Upon high pressure treatment, polymethylesterase (PME) enzyme is liberated and not completely inactivated (which is bound to the cell wall) and brought in close contact with its substrate, the methylated pectin. This caused de-esteri®cation not only during high pressure treatment but also after the release of high pressure (standing time). The pressure treatment also caused partial inactivation of PME. This reaction continued with time even after high pressure treatment
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
29
Fig. 4. Distribution of cell disintegration index with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
Fig. 5. Distribution of relative moisture content with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
and results in time-depended softening of potato tissue. The alteration in pectin resulted in loss of water and soluble solids (or extractable pectin) after high pressure treatment (Stute, Eshtiaghi, Boguslawski & Knorr, 1996). It is reported in the literature that the softening of some fruits and vegetables at atmospheric pressure over a length of time takes place following high pressure treatment (Basak & Ramaswamy, 1998). The variation in cell disintegration index (Zp ) of pressure-treated and untreated potato samples with distance from the centre of the material for dierent dehydration times, is shown in Fig. 4. It may be inferred
from the ®gures that as the dehydration front moved within the sample, it resulted in sudden increases in Zp values. The pro®les of cell disintegration index (Zp ) were dierent for control and the samples treated at 200 and 400 MPa, due to dierence in the degree of cell disintegration with the applied pressure (Fig. 3(a) and (b)). The Zp values of the untreated sample did not change with time and remained zero. The Zp values of the centre layer after 5 h, for the control and sample treated at 200 MPa were 0.18 and 0.28, respectively. In case of samples treated at 400 MPa, the Zp values were 0.82, which showed that the variation in Zp values was due to
30
N.K. Rastogi et al. / Journal of Food Engineering 45 (2000) 25±31
combined eect of osmotic stress owing to osmotic pressure as the dehydration proceeds and high-pressureinduced permeabilisation. The Zp values of all the layers, in the case of pressure treated at 400 MPa, were higher than the control and the sample treated at 200 MPa at the corresponding distances and immersion times. There was a progressive increase in the Zp values as the osmotic dehydration proceeded. The Zp values of the centre layer of potato treated at 400 MPa after 5 h reached 0.24, 0.50, 0.66, 0.75 and 0.82 after 1, 2, 3, 4 and 5 h, respectively (Fig. 4).
The variations in relative moisture (M/M0 ) and solid (S/S0 ) content with respect to the distance from the centre of the material for the pressure treated and control potato samples for dierent treatment times are shown in Figs. 5 and 6. The relative moisture and solid content for control and the sample treated at 200 MPa were not signi®cantly dierent, whereas the moisture content was reduced and solid content increased in the case of samples treated at 400 MPa.
4. Conclusion The cell disintegration index (Zp ) values were close to 1 (total permeabilisation) at the surface in contact with surrounding osmotic solution and as the transition layer (dehydration front) moved towards the centre, it resulted in a sudden increase in Zp values. In case of HHPtreated samples, the Zp values were also high at the centre (which increased with time) which is due to combined eect of high hydrostatic pressure and osmotic stress. The application of high pressure resulted in cell permeabilisation facilitating diusion. The osmotic dehydration of high pressure-treated potato samples was faster than the untreated one due to the combined eect of cell permeabilisation due to osmotic stress (as the dehydration proceeds) and high-pressure-induced permeabilisation.
Acknowledgements The author NKR gratefully acknowledges the support of German Academic Exchange Service (DAAD) for providing a research fellowship at Berlin University of Technology, Germany.
References
Fig. 6. Distribution of relative solid content with respect to distance from the centre of the potato samples (thickness 10 mm) during osmotic dehydration of: (a) control sample; (b) pressure pretreated at 200 MPa and (c) 400 MPa for 10 min.
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